The present invention generally relates to a total contact type photoelectric conversion device and an optical reader using the same, and in particular to a total contact type photoelectric conversion device in which a sheet of paper or the like to be optically scanned is made to slide on the photoelectric conversion device during the scan, and an optical reader using the same. The present invention can be suitably applied to a facsimile machine, a digital copying machine, a digital color copying machine, an optical character reading apparatus, an electronic blackboard and the like.
In general, a total contact type photoelectric conversion device, which is also referred to as a total contact type image sensor, employs a glass plate as a transparent sensor protecting layer. In a conventional optical reader utilizing the total contact type image sensor, a roller for feeding paper to be scanned is positioned so as to make contact with the sensor protecting layer of the image sensor. Paper is wrapped around a part of a peripheral surface of the roller and is made to slide on the sensor protecting layer of the image sensor in a sub-scanning direction. In general, the image sensor has a transparent window and photoelectric conversion elements which are aligned in a main scanning direction. These constituents covered by the transparent sensor protecting layer are positioned under a portion thereof to which a load of the roller is applied. In other words, an optical path region of the image sensor is positioned in a roller load region. Therefore, when the transparent protecting layer is damaged and/or worn away due to the sliding of the paper thereon, there occur problems of a decrease in quantity of light received by the photoelectric conversion elements and a decrease in the modulation transfer function of the image sensor (hereafter simply referred to as MTF).
In order to solve the above problems, conventionally, a transparent wear-resistant layer is formed on the transparent protecting layer. However flaws are introduced in the wear-resistant layer after long-term use, and therefore causes light to be scattered. As a result, the MTF is decreased as in the former image sensor. Additionally, the wear-resistant layer must be transparent and hard. This requirement decreases the degree of flexibility of selecting material for the wear-resistant layer.
FIG. 1 shows an essential part of a conventional total contact type image sensor 30 together with a part of a paper feed roller 10. A sheet of paper to be scanned is partially wrapped around a part of a peripheral surface of the roller 10. The image sensor 30 comprises optically transparent substrate 31, a light screening layer 32, a transparent window 33, a transparent insulating layer 34, photoelectric conversion elements 35, a transparent protecting layer 36 and a transparent wear-resistant layer 37.
The light screening layer 32 is formed on the transparent substrate 31 and is made of an optically opaque and electrically poor conductor. The transparent window 33 is formed in the light screening layer 32 and is used for illuminating the paper 20. The transparent insulating layer 34 is formed on the light screening layer 32 and is filled into the transparent window 33. The photoelectric conversion elements 35 are formed on the transparent insulating layer 34 and are aligned in a main scanning direction parallel to an axial direction of the roller 10. The transparent protecting layer 36 is used for protecting the aligned photoelectric conversion elements 35. The wear-resistant layer 37 is formed on the transparent protecting layer 36 and is used for protecting the transparent protecting layer 36.
Incident light L which enters into the image sensor 30, passes through the transparent substrate 31, the transparent window 33 in the light screening layer 32, the transparent insulating layer 34, the transparent protecting layer 36 and the transparent wear-resistant layer 37, and is reflected on the surface of the sheet of paper 20. The reflected light passes through the transparent wear-resistant layer 37 and the transparent protecting layer 36, and a part of the incident light L reaches the photoelectric conversion element 35, at which the part of the light L is converted to an electric output signal. The paper 20 fed by the roller 10 in a direction of an arrow is pressed on the image sensor 30 within a range I due to the load of the roller 10. This range I is hereafter referred to as a roller load region. In general, a reflection point on the paper is designed to be located at a maximum roller load position which corresponds to an intersection between the paper 20 and a plane passing through the axis of the roller 10 and is perpendicular to the transparent substrate 31. Therefore, the transparent window 33 and the photoelectric conversion elements 35 are positioned within the roller load region I.
The transparent wear-resistant layer 37 is formed in order to prevent the transparent protecting layer 36 from being worn away due to the sliding of the paper 20. A material suitable for the transparent wear-resistant layer 37 must be transparent and hard. However, there is no material for completely satisfying the above requirements. Flaws on the surface of the transparent wear-resistant layer 37 and abrasion thereof cause a decrease in the quantity of light which reach the photoelectric conversion elements 35 and degrades the MTF of the image sensor. Hence, the conventional image sensor cannot stand long-term use.
Additionally, an air gap formed between the paper 20 and the surface of the transparent wear-resistant layer 37 is of the order of approximately 10 .mu.m, which corresponds to the roughness of paper. For this reason, conventionally, the transparent protecting layer 36 must be formed so L as to have a thickness of 70 to 100 .mu.m which is considerably greater than the thickness of the air gap. As a result, the value of the MTF of the conventional image sensor is approximately 0.5 at most.
FIG. 2 shows another conventional photoelectric conversion image sensor 30a together with the part of the roller 20. The illustrated image sensor 30a comprises the transparent substrate 31, the light screening layer 32, the transparent window 33, the transparent insulating layer 34, a transparent passivation layer 38, an adhesive layer 39, the transparent protecting layer 36 and the transparent wear-resistant layer 37. The transparent passivation layer 38 made of silicon dioxide (SiO.sub.2), silicon oxynitride (SiON), silicon nitride (Si.sub.3 N.sub.4) and so on is formed on the transparent insulating layer 34 and the photoelectric conversion elements 35. The transparent protecting layer 36 is a glass plate, which is secured on the passivation layer 38 by the adhesive layer 39. The transparent wear-resistant layer 37 is made of Si.sub.3 N.sub.4.
The incident light L passes through the layers and is reflected on the sheet 20. The reflected light enters the image sensor and reaches the photoelectric conversion elements 35. Because the incident light L and the reflected light are scattered and attenuated, only a part of the incident light L reaches the photoelectric conversion elements 35.
The wear-resistant layer 37 made of Si.sub.3 N.sub.4 has a refractive index of approximately 2.0. The transparent protecting layer 36 of the glass plate has a refractive index of approximately 1.6. The air gap of approximately 10 .mu.m between the wear-resistant layer 37 add the paper 20 has a refractive index of approximately 1.0. It is noted that the refractive index of the air gap is much lower than that of the transparent wear-resistant layer of silicon nitride. This results in an increase of light which is reflected at boundary surfaces between the air gap and the transparent wear-resistant layer 37 and therefore decreases the MTF. Additionally, a great difference in the refractive index between adjacent layers increases the quantity of light reflected at the boundary surfaces therebetween.
FIG. 3 is a view for explaining the above problems. In FIG. 3, a reference numeral 40 denotes an air gap or layer formed between the paper 20 and the transparent wear-resistant layer 37. The refractive indexes of the air layer 40, the wear-resistant layer 37 and the transparent protecting layer 36 are denoted by n.sub.1, n.sub.2 and n.sub.3, respectively. With this layer structure, the following relationship can be obtained in accordance with Fresnel's formula: ##EQU1## where I.sub.0 denotes the quantity of a light component entering the transparent protecting layer 36, and I.sub.1 denotes the quantity of a light component reaching the paper 20. A difference between I.sub.0 and I.sub.1 corresponds to the quantity of light components I.sub.2 reflected at the boundary surfaces.
FIG. 4 is a graph of .beta. vs. the refractive index n.sub.2 of the wear-resistant layer 37 when n.sub.1 =1.0 and n.sub.3 =1.5. It can be seen from the graph that an optimum value of the refractive index n.sub.2 is approximately 1.25, and in this case a value of .beta. equal to 0.98 is obtainable. This means that only 2% of the incident light is reflected on the boundary surfaces between adjacent layers including the air layer. When n.sub.2 =2.0, .beta. is equal to 0.87, and therefore more than 13% of the incident light is reflected on the boundary surfaces between the layers. That is, the light components I.sub.2 are increased and thereby the MTF is decreased by about 20%, compared to the case where n.sub.2 =1.25.